1. Field of the Invention
This invention relates to a focus detecting apparatus suitable for a single-lens reflex camera.
2. Description of the Related Background Art
As one of the focus detecting systems for single-lens reflex cameras, there is known the image correlation system. FIGS. 7 and 8 of the accompanying drawings illustrate focus detection by the image correlation system, FIG. 7 showing the optical path of a focus detecting optical system, and FIG. 8 showing the light amount distribution on a light detecting element (hereinafter referred to as a sensor) in which photo-electric elements are arranged in the form of a line. In these figures, the reference numeral 1 designates a film-equivalent surface, the reference numeral 2 denotes a field lens, the reference numeral 3 designates secondary imaging lenses, the reference numeral 4 denotes sensors, and the reference numeral 5 designates a photo-taking lens.
An on-axial light beam emitted from a point source of light P on the optic axis a (in the figure, parallel to the x-axis) of the photo-taking lens 5 is imaged near the film-equivalent surface 1 through the photo-taking lens 5. The object light is transmitted through the field lens 2, is converged by the secondary imaging lenses 3 and is projected onto the sensors 4. Here, the secondary imaging lenses 3 have their focal length and arrangement set so that the image on the film-equivalent surface 1 is imaged on the sensor 4 at a predetermined magnification. Also, the secondary imaging lenses 3 are disposed as one set of two at locations symmetrical with respect to the optic axis of the photo-taking lens 5, and cause part of the object light to be imaged on the one set of two sensors 4. The sensors may be a row of sensors divided into two areas.
FIG. 7A shows a state in which the positional relation between the object P (which is the point source of light) and the photo-taking lens 5 and the film-equivalent surface is in the in-focus state. FIG. 8A shows the light amount distributions on the sensors 4 at this time, and a light amount distribution having a sharp peak substantially at the central position of each sensor A, B is created, and the interval between the peak positions is l.sub.0. l.sub.0 is an independent value determined by the construction of the focus detecting optical system.
FIG. 7B shows the optical path in the forward focus state in which the photo-taking lens 5 has been moved to the object side, and the light amount distributions on the sensors 4 present distributions low in the degree of sharpness, as shown in FIG. 8B, and the internal l between the peak positions of the light amount distributions on the sensors is narrower than the interval l.sub.0 in the in-focus state. The difference .DELTA.l(=l-l.sub.0) between these peak-position intervals corresponds to the defocus amount and therefore, focus detection is accomplished using this amount .DELTA.l.
FIG. 7C shows the optical path in the rearward focus state after the photo-taking lens 5 has been moved to the film surface side, and the light amount distributions on the sensors 4 present distributions low in the degree of sharpness, as shown in FIG. 8C, and the interval l between the peak positions on the sensors is wider than the interval l.sub.0 in the in-focus state. The difference .DELTA.l (=l-l.sub.0) between these intervals corresponds to the defocus amount and therefore, focus detection is accomplished using this amount .DELTA.l. Also in the case of an ordinary object having a brightness distribution, the correlation between the object images on the sensors is taken and focus detection is accomplished from the amount of deviation l between the two images. Also, the field lens 2 is disposed near the film-equivalent surface (including the film-equivalent surface), and is set to such a focal length that the positional relation between the exit pupil P of the photo-taking lens 5 and the entrance pupils of the secondary imaging lenses 3 satisfies the imaging relation. Also, a stop 6 usually having two openings is disposed near the pupil positions of the secondary imaging lenses 3, and this stop is set to such a size that two light fluxes transmitted through the openings thereof are separated and not eclipsed in the photo-taking lens 5.
However, in the above-described focus detecting system, when an attempt is made to effect focus detection of an object lying at a location in the photo-taking picture plane which is off the optic axis of the lens, if the disposition of the sensors is done arbitrarily, the openings of the stop at the focus detecting position must be made small and the interval between the openings of the stop must be made narrow so that the focus detecting light flux entering the sensors may not have its focus detecting performance deteriorated under the influence of the residual aberrations of the photo-taking lens or the focus detecting light flux may not be eclipsed in the photo-taking lens, and this has led to the disadvantage that the focus detecting performance is deteriorated.
FIG. 9 of the accompanying drawings illustrates how the focus detecting light flux is influenced by the aberrations of the photo-taking lens, and shows the optical path of the focus detecting optical system. The reference numeral 1' designates the film-equivalent surface, the reference numeral 2 denotes the field lens, the reference numeral 3 designates the secondary imaging lenses, the reference numeral 4 denotes the sensors, the reference numeral 5 designates the photo-taking lens, the reference numeral 6 denotes the stop, and the reference characters 9A and 9B designate focus detecting off-axial light fluxes.
An object P.sub.1 is in the direction -y relative to the optic axis of the photo-taking lens 5 in a plane x-y containing the optic axis, and the object light is transmitted through the photo-taking lens 5, whereafter it is imaged at a position of height h in the direction of the y-axis on the film-equivalent surface 1'. The stop 6 and the secondary imaging lenses 3 divide the object light into two and direct the divided light fluxes onto the two sensors 4, and the direction of division of the light fluxes is the direction of the y-axis and is coincident with the direction of deviation of the object image from the optic axis. At this time, the two divided focus detecting light fluxes 9A and 9B are asymmetrical with respect to the optic axis of the photo-taking lens 5 and therefore, aberrations having asymmetry are created in the respective object images on A and B of the sensors 4. As a result, in an apparatus wherein the correlation between the images on A and B of the sensors 4 is taken to thereby accomplish focus detection, it becomes difficult to take the correlation between the two images and the focus detecting performance is deteriorated.
FIG. 10 of the accompanying drawings shows the vignetting in the opening of the stop 6 which corresponds to the focus detecting light flux 9B. The solid line indicates the opening of the stop 6, and it is set to a shape in which no vignetting occurs when the image height h=0 [mm] (which corresponds to a in FIG. 10). As the image height h becomes higher to 6 [mm] (the dotted line b in the figure), 9 [mm] (the dot-and-dash line c in the figure) and 12 [mm] (the broken line d in the figure), eclipse of the light flux occures from the direction -y in the figure. Here, the area in the direction+y of the areas demarcated by the respective lines is an effective light flux area, and as the image height becomes higher, the light fluxes entering the sensors become less and the performance of the brightness of the focus detecting apparatus is reduced Also, as the image height becomes higher, the position G of the center of gravity of the light flux comes near to the optic axis of the photo-taking lens (the x-axis in the figure) and the distance L between the position G of the center of gravity of the light flux and the optic axis of the photo-taking lens becomes smaller. Here, the value of 2*L is a value corresponding to the length of the base line in triangulation, and if L becomes smaller, there arises the difficulty that the focus detecting performance is reduced.
On the other hand, an apparatus designed to effect focus detection for an object lying at an off-axis position off the optic axis of the photo-taking lens is known from Japanese Laid-Open Patent Application No. 62-47612 and Japanese Laid-Open Patent Application No. 62-189415. Particularly, one of the examples of the arrangement of sensor arrays disclosed in Japanese Laid-Open Patent Application No. 62-189415 is such that respective sensor arrays disposed off the axis are provided so as to be symmetrical with respect to a meridian centered at the optic axis. However, the other examples of the arrangement are such that the off-axis sensor arrays are not symmetrical with respect to the radius, and it is apparent that these examples do not take into account the aberrations caused in the imaging light flux by the objective lens.